1. Field of the Invention
This invention relates to the production of a silicon nitride ceramic which contains silicon carbide whiskers, silicon nitride powders, or both by a method which includes a reaction bonding step and a sintering step.
2. The Published Art
U.S. Pat. No. 4,356,136, granted Oct. 26, 1982 to Mangels, discloses that a reaction bonded silicon nitride ceramic (without silicon carbide whiskers) can be packed in silicon nitride powder and a densification aid, and densified by a two step firing procedure in a nitrogen atmosphere. The first step is in a low pressure nitrogen atmosphere while the second is in a high pressure nitrogen atmosphere. Example 1 of the patent discloses a first step firing in 15 psi. nitrogen to 1875.degree. C. and a second step firing in 300 psi. nitrogen to 1925.degree. C.
U.S. Pat. No. 4,351,787, granted Sept. 28, 1982 to Martinengo et al., discloses that a shape can be pressed from a mixture of silicon particles and a sintering aid (again, without silicon carbide Whiskers), that the shape can be nitrided to produce a reaction bonded shape, and that the reaction bonded shape, after it has been embedded in a protective powder can be densified y firing in a nitrogen atmosphere. In Example 1, the protective powder is composed of Si.sub.3 N.sub.4, BN, MgO and Fe and the firing is to 1800.degree. C., final nitrogen pressure 750 Torr.
U.S. Pat. No. 4,687,655, granted Aug. 18, 1987 to Hunold et al., discloses (Example 6) that ceramics which do not include silicon carbide whiskers, but having a silicon nitride content of 99.1 percent, 98 percent of theoretical density, can be produced by cold isostatic pressing of a mixture of silicon powder, yttrium oxide powder and aluminum oxide powder, and subjecting the shapes which are produced to a two-stage firing in a high pressure nitrogen atmosphere. The first stage of the firing was to a maximum temperature of 1270.degree., nitrogen pressure from 50 to 80 MPa, while the second stage was to a maximum temperature of 1800.degree., nitrogen pressure from 80 to 150 MPa. The patent also states that cycles comparable to the Examples would last from 100 to 140 hours in the case of nitridation with nitrogen or nitrogen/hydrogen mixtures under normal pressure.
Various suggestions have also been made or ceramic composites which include silicon carbide whiskers. For example, composites comprising Si.sub.3 N.sub.4 reinforced with SiC whiskers are disclosed by Lundberg et al., Am. Ceram. Soc Bull., 66 [2]330-33 (1987). The Lundberg et al. reference discloses production of the composites by cold isostatic pressing of a batch to produce bars, nitridation of the bars, and sintering of the nitridated bars. The batch was composed of from 70 to 100 percent of a matrix material and up to 30 percent of SiC whiskers. The matrix material was composed of 55 percent of submicrometer Si, 37 percent of Si.sub.3 N.sub.4, 6 percent of Y.sub.2 O.sub.3 and 2 percent of Al.sub.2 O.sub.3. A two-step nitridation was used, 1 hour at 1200.degree. followed by 3 hours at 1350.degree.. The nitrided bars were were either pressureless sintered at 1850.degree. or hot isostatically pressed in argon at 1700.degree. .
It has also been suggested (see, for example, U.S. Pat. No. 4,500,482, granted Feb. 19, 1985) that a mixture of silicon, silicon nitride and an organic binder (without silicon carbide whiskers) can be molded to a desired shape, nitrided, and hot isostatically pressed to produce a silicon nitride ceramic (see, for example, GB PS 1,546,928, May 31, 1979) and that a molding (without silicon carbide whiskers) can be produced from silicon powder and sintered in an inert atmosphere to 60-75 percent of theoretical density prior to nitriding to form silicon nitride.